This invention relates to a process and apparatus wherein an olefin is polymerized in a particle form process in a liquid diluent to form a liquid slurry in a polymerization zone and the particles are separated from the slurry by a liquid cyclone separator in heat exchange relationship with the polymerization slurry to receive heat therefrom, the resulting clarified liquid is directed back to the polymerization zone and the concentrated slurry is withdrawn from the cyclone.
The separation of the solid particles of polymer formed in a particle form process is a problem because it is necessary that the solid particles be removed from the liquid for further processing and the liquid returned to the polymerization zone for further polymerization because this liquid contains in addition to the liquid diluent the unreacted monomer or monomers. Several prior patents disclose various ways of separating the solid particles and these are as follows.
Phillips Petroleum Co. U.S. Pat. No. 3,242,150 discloses a continuous loop polymerization reactor having a receiving zone or settling leg at the bottom of the vertical reactor into which the solid particles gravitate. Although this provides the separation of the solid particles such a system poses certain operating problems in view of the mechanical complexities of the settling leg system, the risk of polymer agglomeration and resultant blockage of the settling leg, the extreme sensitivity of the efficiency of the system to the particle size and bulk density of the slurry, and the circulation rate of the polymer particles in the reactor as they pass over the entry to the settling leg.
Other processes have been described to improve the slurry removal system from a particle form polymerization reactor, such as the use of an external liquid cyclone (hydroclone) as disclosed in U.S. Pat. Nos. 3,816,383 and 4,007,321. However, these processes require complicated piping and valving to and from the hydroclone and are dependent upon maintaining and controlling a pressure gradient across the hydroclone and in some cases returning a recycle flow back to the polymerization reactor to prevent plugging of the system. In addition, since all the piping, valving and vessels are external to the reactor system, they must be heated and insulated to control the slurry temperature near the reactor polymerization temperature to prevent the precipitation of the low molecular weight soluble polymer from the reactor diluent resulting in coating and pluggage of lines and vessels if the temperature drops too low or to prevent solubilization of the slurry polyolefin particles with the resultant formation of polymer rope and subsequent plugging in the flash tank and/or drier if the temperature rises too high.